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MSA: A key technology for the evolution of future wireless networks 文档密级
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I Executive Summary
Operators worldwide must prepare for massive mobile broadband (MBB) network traffic
growth as the industry moves away from voice-and-text, pure-pipe services to an era of data
diversity and new vertical market revenue streams. Total network traffic is widely forecasted to
increase at least 1,000 times over the next decade.
Meeting such unprecedented growth in demand for mobile services will require taking
advantage of more than one wireless network technology to ensure a top user experience can be
provided, maintained and continuously enhanced with interworking combinations of macro and
small cells. Multi-Stream Aggregation (MSA) leverages the centralized integration of multiple
radio access technologies (RAT), carriers and intra-carrier ports to help grow network capacity
needed for providing a No-Edge network user experience.
A combination of a layered network architecture and MSA in future networks will enable
users to enjoy high-speed and highly reliable mobile services anytime and anywhere. A layered
network architecture includes a host layer which provides a basic user experience and ensures
reliable network coverage, and a boost layer which increases network capacity and provides the
best possible user experience. MSA is a key technology for integrating the capabilities of both of
these layers.

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II Current network challenges
For current network deployments, different RAT standards such as UMTS, LTE and WiFi
are typically deployed and managed independently and access the network through their
respective core network gear. With such a “mono-layer” architectural approach, User Equipment
(UE) can only utilize data services from a single cell site through a single RAT at any one time
(see Figure 1 below). This results in poor resource utilization, and redundant network
infrastructure investment.
Internet
WiFi AC
3G CN
UMTS
F1
LTEF2
AP
F3
LTE EPC
Macro cell
Macro cell
Small cell
Small cell
Figure 1:“Mono-layer” network architecture
Heterogeneous network (HetNet) deployments involving coordinated macro and small cell
coverage have been a common means of improving capacity for a mono-layer approach. But as
the number of deployed cells increases, so does the number of cell edges. At these cell-edges, end
user experience can be significantly impacted by frequent handover (HO), increased HO failure
rate, and low throughput.

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1. Handovers
With dense network deployments, frequent and/or ping-pong HOs have become major
obstacles to providing a top-quality user experience. Figure 2 below gives an illustration of the
geographical distribution of Reference Signal Received Power (RSRP) before and after small cell
deployment. As shown under the same red line area, before small cell deployment, UE signal
attenuation becomes slower with increases in distance, but after small cell deployment, UE signal
strength near the small cell is significantly improved but the signal weakens considerably with
distance, especially around street corners.
Small
cell
Macro
cell
Figure 2: RSRP geographic distribution before and after small cell deployment
Due to small cell fast channel fading and interference, HO failure rate for HetNet is generally
higher than that for macro cell networks, especially for HOs from small cells to macro cells (S2M
in HetNet, see Figure 3 below).
Figure 3: HO failure rate comparisons

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2. Interference
Small cells deployed within a macro cell’s coverage are often subjected to intra-frequency
interference from the macro cell, which in turn contracts a small cell’s coverage depending on
how close its position is to the center of the macro cell. Figure 4 below shows how a small cell’s
coverage changes when deployed in different locations. Near the macro cell’s edge, the small
cell’s coverage may span over 100 meters from one small cell edge to the other, but closer to the
macro cell’s center the small cell’s coverage may only reach as far as 10 meters. What’s more,
intra-frequency interference also significantly hinders UE throughput.
Macro cell
Small cell
RSRP
[dBm]
Distance
Figure 4: Small cell coverage contraction due to intra-frequency interference

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3. Resource Utilization
Inefficient and inflexible utilization of network resources is a major operator concern given
that traffic load imbalances between macro cells and small cells are prone to sudden and dynamic
changes. For traditional “mono-layer” HetNets, resources can’t be shared between different cell
sites, causing stark differences in quality of experience (QoE).
Figure 5 below illustrates a scenario whereby four UEs (UE1, UE2, UE3 and UE4) are being
provided mobile services by a macro cell while UE5 and UE6 are being separately provided
mobile services by two small cells.
UE 2 UE 3 UE 4 UE 5 UE 6
Macro cell Resource Small cell 1 Resource Small cell 2 Resource
UE 1
UE 2
UE 3
UE 5
UE 4
UE 6
UE Throughput (Mbps)
Macro cell
Small cell 1
Small cell 2
UE 1
Figure 5: Uneven distribution of UE experience
The macro cell and small cells share the same total resources. Throughput for UEs served by
a macro cell is typically lower since traffic loads for macro cells are on average heavier due to the
need to provide simultaneous services to multiple UEs over a greater area, while throughput for
cell-edge UEs, such as UE3 and UE4, is especially low due to increased channel propagation
distance and interference. In general, only a few UEs are served by small cells due to limited
coverage, so UE throughput inside a small cell’s coverage, such as UE5 and UE6, is comparably
higher due to more available resources. A consistent QoE between macro cells and small cells is a
considerable challenge to providing a seamless No-Edge network user experience.

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III MSA: A key to MBB network evolution
MSA leverages centralized integration of multiple RATs, carriers, and intra-carrier ports to
resolve the aforementioned operator pain points and provide a significant cell-edge throughput
improvement for a No-Edge network user experience. A combination of a layered network
architecture and MSA in future networks will enable users to enjoy high-speed and highly reliable
mobile services anytime and anywhere.
A layered network architecture includes a host layer and a boost layer (see Figure 6 below).
The host layer ensures a QoE baseline and provides reliable MBB network coverage and data
services. A “Host Link” is established on the host layer to enable UE signaling and data
transmission. The function of the boost layer is to increase network capacity and provide the best
possible user experience by using all available resources, and establishing “boost links” to provide
UEs with enhanced data transmission.
MSA is a key technology for integrating the capabilities of both of these layers to further
enhance user experience and network capacity. MSA has been standardized since 3GPP Release
10 and has been a hot topic for the current Release 12.
Host
Layer
SRC/
BBU pool
Boost
Layer
F1
Core Network
Macro cell Macro cell
F1
WiFi AP
Inter-RAT
MSA
F2
Small cell F3
Inter-frequency
MSA
Small cell
Small cell
F1
F1
Intra-frequency
MSA
Figure 6: Host/boost network layering with MSA
As a RAN element, a centralized BBU pool or a SRC (Single Radio Controller) can be used
as a central node to implement MSA while performing unified control functions such as network
layering, traffic steering, and coordinated scheduling.
Figure 7 below shows the performance gains made possible by the combination of a layered
network architecture and MSA. In a traditional “mono-layer” HetNet without MSA, a handover is

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triggered when a UE moves between a macro cell and a small cell, which might affect the UE
experience especially in cases of call drops (as shown by blue curve). However, after introducing
the combination of a layered network architecture and MSA, handovers and interference are
effectively eliminated, system resources are fully utilized, which will significantly enhance UE
experience .
Data Rate
[Mbps]
Traditional Hetnet
Intra-frequency MSA
Inter-frequency MSA
Inter-RAT MSA
No Edge
Macro cell
Small cell
Figure 7: Performance gains from the combination of a layered network architecture and MSA

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1. Host Layer ensures a user experience baseline
The host layer’s primary function is to eliminate network performance issues related to HOs
and interference to provide an unwaveringly reliable QoE baseline.
1.1 Handovers
A drastic reduction in HOs can be achieved for both intra-frequency and inter-frequency
scenarios.
 Intra-frequency: All small cells within a macro cell’s coverage share the same cell ID as
the macro cell, so no intra-frequency HOs will be triggered when a UE moves within the
macro’s coverage.
 Inter-frequency: A UE is always anchored to the macro cell. Wherever it goes within the
coverage area of the macro cell, a host link between the UE and macro cell is maintained
so that no inter-frequency HO is triggered.
1.2 Interference
With a layered architecture in place, interference can be further divided into intra-layer and
inter-layer interference.
 Intra-layer interference: Coordinated scheduling of neighbor cell resources through the
host layer can be used to effectively minimize intra-layer interference, which is especially
useful for interference-prone UEs.
 Inter-layer interference: Time-frequency resource separation can effectively minimize
inter-layer interference. In this way, a portion of time-frequency resources are devoted to
SFN (Single Frequency Network) transfers from different ports on the host layer to
achieve the best coverage, while all remaining resources are efficiently allocated through
spatial multiplexing between the host layer and boost layer to achieve the best utilization
efficiency.
The host layer effectively ensures mobile service continuity by eliminating HOs and
improving throughput by reducing interference to ensure a QoE baseline.

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2. Boost layer provides best possible user experience
The boost layer enhances user experience beyond a baseline, and MSA is a key technology
that works with the boost layer and host layer to make this possible. As for its different application
scenarios, MSA can be intra-frequency, inter-frequency and inter-RAT.
2.1 Intra-frequency MSA: Utilizing all available intra-carrier ports
In a mono-layer HetNet architecture, a single UE is served by a single port, which is an
inefficient usage of system resources. After introduction of intra-frequency MSA, a single UE can
dynamically connect to one or more of the best available ports to achieve the best possible UE
experience. With intra-frequency MSA, data transmission is made possible without the need for
additional signaling, thus maximizing utilization of system resources even when a UE moves
between different cell IDs. This resolves issues inherent to more efficient resource utilization,
making for a more geographically consistent UE experience.
Figure 8 below illustrates the benefits of intra-frequency MSA for providing a more evenly
distributed user experience. UEs located on cell-edges (UE3 and UE4) are served by both the
macro cell and small cell for increased throughput while the other UEs in the macro cell (UE1 and
UE2) can also increase throughput due to greater allocation of resources. UEs located inside a
small cell’s coverage (UE5 and UE6) share resources with cell-edge UEs but this has a trivial
impact on user experience since the traffic loads of small cells are light in most cases.
UE 1
UE 2
Macro cell
Host
Layer
F1
Small cell 1
UE 5
UE 3
UE 4
UE 6
Small cell 2
F1
F1
Boost
Layer
UE 1 UE 2 UE 3 UE 4 UE 5 UE 6
UE Throughput
(Mbps)
Resource Pool
UE2 UE3 UE4 UE5 UE6
Macro cell Resource Small cell 1 Resource Small cell 2 Resource
UE 1
UE 2
UE 3
UE 5
UE 4 UE 6
UE Throughput
(Mbps)
Macro cell
Small cell 1
Small cell 2
Host link
Boost link
UE1
Figure 8: Without and with intra-frequency MSA resource utilization comparison
Intra-frequency MSA utilizes several advanced algorithms that enable cell-edge throughput

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gains of 200%, including:
 Interference management with Coordinated Scheduling Power Control (CS-PC)
 Adaptive Coordination Load Balancing (CLB) for maximum spectrum utilization
 Coordinated Multi-Point transmission and reception (CoMP) for dynamic point selection
and joint transmission tracking for channel fade variations and unbalanced traffic
distribution
2.2 Inter-frequency MSA: Utilizing all available carriers
In a mono-layer HetNet architecture, when a UE moves between a macro cell using one
carrier and a small cell using another carrier, an inter-frequency HO will occur that can adversely
affect user experience. However, with inter-frequency MSA, UE is always anchored to the macro
cell through a host link even while dynamically connecting to other carriers through boost links
for enhanced data transmission. In other words, MSA with different carriers further enhances user
experience and increases network capacity.
Inter-frequency MSA is applicable for both ideal and non-ideal backhaul cases between
macro and small cells. In ideal cases, the backhaul link latency between a macro and small cell is
negligible, but in non-ideal cases it isn’t. And now, MSA utilizing different carriers in non-ideal
backhaul scenarios has become a hot topic for 3GPP Release 12.
2.3 Inter-RAT MSA: Utilizing all available RATs
Inter-RAT MSA utilizes different RATs to enhance user experience. In this case, the host
layer can be UMTS or LTE, while the boost layer can be LTE or WiFi. For a LTE-WiFi scenario,
LTE acts as the host layer and WiFi acts as the boost layer. The former provides basic mobile
services to the user, with an LTE host link remaining connected with the UE to guarantee a QoE
baseline. WiFi then enhances user experience by providing a boost link between the UE and WiFi
Access Point (AP) to boost data transmission rates.
In most cases, downlink data traffic volume exceeds that of the uplink, but allocated
resources for downlink and uplink are in approximate symmetry, so other resources are needed to
assist downlink data transmission. Furthermore, WiFi uplink suffers from more serious

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performance issues related to access collision, hidden terminals, and Quality of Service (QoS) that
get worse as the number of connected UEs increases. Based on the above two considerations, a
more efficient transmission scheme is to use a WiFi network for downlink data transmission alone
(see Figure 9 below). In this scenario, SRC flexibly steers traffic from the host link to the boost
link through WiFi according to the channel quality, network load, and interference to boost UE
experience and significantly increase network capacity.
UE
eNB SRC
AP
CN
Server
Internet
UL
DL
Figure 9: Inter-RAT MSA with WiFi for downlink data transmission alone

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IV Conclusion
The continuing widespread growth, diversification and availability of HD mobile services
will no doubt severely strain capacity for future MBB networks. In order to meet growth in
demand for mobile services, MSA leverages the centralized integration of multiple RATs, carriers,
and intra-carrier ports to resolve all aforementioned operator pain points and provide cell-edge
throughput improvements of 500%.
A combination of a layered network architecture and MSA in future networks will enable
users to enjoy high-speed and highly reliable mobile services anytime and anywhere, in which a
host layer provides reliable network coverage and ensures a QoE baseline, and a boost layer
increases network capacity and provides the best possible user experience. MSA is a key
technology that works with the host layer and the boost layer to make this possible.
At present, all of the aforementioned technologies and solutions have been prototyped on
existing product platforms. Significant QoE improvements that help realize a No-Edge network
user experience have been successfully demonstrated via a combination of host/boost network
layering and MSA outfield trials.